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.\" $FreeBSD: src/lib/libc/gen/makecontext.3,v 1.4 2002/12/19 09:40:21 ru Exp $
.\"
.Dd September 10, 2002
.Dt MAKECONTEXT 3
.Os
.Sh NAME
.Nm makecontext , swapcontext
.Nd modify and exchange user thread contexts
.Sh LIBRARY
.Lb libc
.Sh SYNOPSIS
.In ucontext.h
.Ft void
.Fo makecontext
.Fa "ucontext_t *ucp"
.Fa "void \*[lp]*func\*[rp]\*[lp]\*[rp]"
.Fa "int argc" ...
.Fc
.Ft int
.Fn swapcontext "ucontext_t *oucp" "const ucontext_t *ucp"
.Sh DESCRIPTION
The
.Fn makecontext
function
modifies the user thread context pointed to by
.Fa ucp ,
which must have previously been initialized by a call to
.Xr getcontext 3
and had a stack allocated for it.
The context is modified so that it will continue execution by invoking
.Fn func
with the arguments (of type
.Ft int )
provided.
The
.Fa argc
argument
must be equal to the number of additional arguments provided to
.Fn makecontext
and also equal to the number of arguments to
.Fn func ,
or else the behavior is undefined.
.Pp
The
.Fa "ucp->uc_link"
argument
must be initialized before calling
.Fn makecontext
and determines the action to take when
.Fn func
returns:
if equal to
.Dv NULL ,
the process exits;
otherwise,
.Fn setcontext "ucp->uc_link"
is implicitly invoked.
.Pp
The
.Fn swapcontext
function
saves the current thread context in
.Fa "*oucp"
and makes
.Fa "*ucp"
the currently active context.
.Sh RETURN VALUES
If successful,
.Fn swapcontext
returns zero;
otherwise \-1 is returned and the global variable
.Va errno
is set appropriately.
.Sh ERRORS
The
.Fn swapcontext
function
will fail if:
.Bl -tag -width Er
.It Bq Er ENOMEM
There is not enough stack space in
.Fa ucp
to complete the operation.
.El
.Sh SEE ALSO
.Xr setcontext 3 ,
.Xr ucontext 3
